Antennas with variable electrical tilt (VET) functionality are known in the art. These antennas, which are used in cellular networks, enable network operators to electrically tilt the elevation beam pointing direction of the antenna by manually rotating a knob or translating a shaft on the exterior of the antenna. The knob or shaft is linked to phase shifters inside the antenna that convert the mechanical rotation or translation of the shaft to phase changes in the radio frequency beam forming network inside the antenna. Changes in phase between radiating elements inside the antenna cause the beam emitted from the antenna to tilt up or down relative to mechanical boresite of the antenna. An example of a cellular base station antenna demonstrating VET technology is depicted in U.S. Pat. No. 7,068,236, which is incorporated by reference.
Beam tilt adjustment is needed in cellular networks to reduce signal propagation between sites in the network in order to minimize signal interference and to maximize network capacity. Antennas with VET functionality allow network operators to make accurate tilt adjustments at a cell site without mechanically tilting the antenna and without changing the visual appearance of the site. Antennas with VET functionality typically include some sort of tilt indicator to provide visual feedback of the antenna electrical tilt setting to a person located at the antenna to inspect the antenna or to manually make the tilt adjustment.
Remote electrical tilt (RET) antennas are also known in the art. RET antennas incorporate an electro-mechanical actuator attached to or installed inside of the antenna to rotate the knob or translate the shaft on a VET antenna. This enables the electrical tilt of the VET antenna to be controlled from a remote location, eliminating the expense of hiring a rigger to climb the tower and manually adjust the electrical tilt of the antenna beam.
This conventional configuration of RET actuators is shown in FIG. 1, in which a tri-band antenna 4 includes three self contained, separately removable RET actuators 5a-c, one for each operational frequency band of the antenna. Each RET actuator is a self contained electro-mechanical device with lightning protection circuits, communications circuits, a motor, motor control circuits, power control circuits and a motor position sensor contained within a single enclosure. For antennas designed to operate over multiple frequency bands, multiple, stand-alone RET actuators are attached to or inserted inside of the antenna housing. Cable assemblies are connected between the RET actuators to provide power and signaling to the multiple RET actuators for that antenna. This design approach is expensive due to the cost of the external cable assemblies and the redundant electronic components used by multiple RET actuators. In addition, removable RET actuators must be configured to physically align with and receive the phase shifter shafts for each beam of the multi-beam antenna, which requires a different removable RET actuator configuration for each antenna with a different phase shifter shaft configuration.
The locations of the phase shifter adjustment knobs or shafts on a typical multi-band RET antenna are constrained by the physical size and shape of the RET actuators and by their attachment mechanisms. The phase shifter adjustment knobs or shafts must be spaced far enough apart to allow the multiple RET actuators to be attached to the antenna without mechanical interference. The knobs or shafts must also be spaced far enough apart to provide room for the RET actuator mounting hardware and to provide access for the tools used to install the mounting hardware. As a result, the location of the phase shifter adjustment knobs or shafts on the antenna are often determined by the geometry of the RET actuators and not by the optimum phase shifter placement inside the antenna. These constraints increase the mechanical complexity of the RET antenna design and increase the development time and costs for new antenna models.
There is, therefore, a continuing need for a RET antenna that can be produced with fewer electronic parts for lower cost and that reduces mechanical constraints on the phase shifter drive shaft locations inside the antenna.